1. Field of Invention
The inventions disclosed in this document are devices, techniques, and methods to suppress stray light in an optical system. More specifically, the invention suppresses stray light in a LCoS based prism assembly/kernel and, by doing so, improves the contrast ratio of the projected video image.
2. Discussion of Background
FIG. 1 is a drawing of an LCoS based video projection system. Stray light is one factor that limits the contrast ratio produced by a LCoS based video projection system. Stray light can be generated within the prism assembly/kernel in several ways including:                Diffraction from the microdisplay.        Non-specular reflection from the surface of the microdisplay.        Reflections from interfaces between the components in the optical path.        Scattering from imperfections on the optical surfaces, in the thin film coatings or within the bulk materials.        
Some of the stray light inevitably exits the kernel to be included in the projected image. The stray light adds light intensity but not content to the video image. In current light engine designs and applications, this can be a significant contrast ratio-limiting factor. Techniques used to suppress stray light include:                The use of picture frame baffles.        Removing light rays that impact and scatter/reflect from the walls of the prism assembly. The removal is accomplished by either absorbing or transmitting the stray light rays.        
However, the present inventors have determined that, in actual light engine applications, these techniques are insufficient. And, the present inventors have realized the need for further and better suppression of stray light.
The LCoS based video projection system also utilizes a number of optical components, such as beam splitting elements, waveplates, dichroics, etc.